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11. PHYSICAL INTERPRETATION OF EQ. (zi)Equation (21) was obtained by dimensional analysis of the equationsof conservation and the necessary boundary conditions, and by the conceptof an eddy thermal diffusivity, which was introduced without looking into thedetailed mechanism of boiling heat transfer. To see how the heat is beingtransferred, it is convenient to write Eq. (21) in the following form:NA(icT)3/2 -nXYq = C[pcpGs[ H e -inQuantities inside the first bracket on the right side of this equation can beconsidered as proportional to the heat exchange between saturated andsuperheated liquid near the heater as propelled by the growing bubbles,while those inside the second bracket as proportional to the velocity ofthis exchange. This confirms, at least in some respect, the "vapor-liquidexchange" mechanism which was recently proposed by Forster and Grief.(3)Equation (21) can also be written in the form p ,ywhere quantities with a bar denote their temporal mean values and theprime indicates the deviation from the mean value Thus G' is the fluctu-ating temperature and v' the fluctuating velocity component in the directionnormal to the heating surface. It follows that the boiling heat transfer canbe solved according to Prandtl missing length concept, and an analogybetween momentum and heat transfer in boiling can be establishedquantitatively.12. MAXIMUM NUCLEATIONThe value of the nucleation function increases at the increase ofsuperheat, but it has a limit as the exponential function approaches tounity. This limit should represent the maximum rate of bubble generationfrom unit area of the heating surface for a given system. Beyond thislimit, further increase of superheat will cause instability of vapor bubbles,and the process of heat transfer changes from nucleate boiling to partialfilm boiling, as pointed out previously. For example, in the case of boilingof benzene from a chromium-plated surface in a pool (see Fig. 3) this con-dition would occur at the superheat of about 80"F under a pressure of oneatmosphere and of about 453F at a pressure of 115 psia. Instead, in thecase of boiling of benzene, from a rough surface (see Fig. 5) this conditionwould occur at a considerably larger superbeat, for the value of n is Largerthan that of smooth surface, as can be seen from Table II. In reality, how-ever, the first critical condition occurs, in general, at a superheat much